The use of turbochargers to increase the air intake of internal combustion engines is a common, well known mean to increase engine output. In many conventional turbochargers the compressor wheel is driven at high speeds or revolutions per minute. For example, many turbocharger wheels rotate in the range of about 100,000 to 150,000 revolutions per minute. This high speed of the rotating blades causes a high frequency noise to be emitted therefrom. When such turbocharged engines are used in vehicular applications such as a truck, the noise can be very annoying and distasteful to the operator and by-standers. The use of insulation in cabs and in engine compartments has greatly reduced the amount of noise emitted from the turbochargers that reaches the operator and by-standers. To date, such noise reduction packages have managed to keep the objections by the operator and by by-standers to an acceptable level. However, certain performance improvements in turbochargers have increased the noise emitted therefrom above the normal level of acceptability by the operator and by-standers. Some examples of approaches to widening the performance band of turbochargers include variable geometry guide vanes and vaned diffusers, turbine bleed devices and valves, casing treatments and the addition of features such as axial and circular grooves.
One such example is disclosed, in U.S. Pat. No. 4,743,161 issued to Frank B. Fisher et al. on May 10, 1988. The goal of this enhancement is to allow operation over a wider speed and load range and also enable higher torque at lower engine speed. What is accomplished is a broadening of the high efficiency range between surge conditions and choke conditions. Surge being where a turbocharger/compressor/engine system is on the edge of instability and stall. Choke conditions being where the system's air requirements exceed the compressor's maximum flow capacity. In this patent, an inducer recirculation groove or bypass is disclosed. The bypass accomplishes two things; increases choke flow by drawing extra air into the stage after the compressor impeller throat, and reduces the flow at which surge occurs at all speeds by joining different parts of the compressor stage with bypass flow. The bypass includes a simple circumferential slot connecting a point along the shroud with a secondary inlet. The bypass produces a positive differential pressure on the inlet at choke and a negative differential pressure on the inlet at surge. The inducer recirculation groove has been found to increase the amount of noise emitted therefrom since the groove connects a point along the shroud with a secondary inlet. Thus, a secondary line of sight or path for the sound waves to pass therealong is constructed when using the inducer recirculation groove.
The problems mentioned above has caused increased negative comment by operators and by-standers. The problems have further caused manufacturers to consider alternatives to turbochargers and variations to noise reduction systems.
The present invention is directed to overcoming one or more of the problems as set forth above.